Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product, such as light generation and electrical signal amplification. The goal of efficient thermal design is to lower the operating temperature of such electronic or optoelectronic device while maximizing performance (power and speed) and reliability. Examples of such devices are microwave transistors, light-emitting diodes and lasers. Depending on the frequency of operation, power requirements, and specific application, these devices have been conventionally made on silicon, gallium arsenide (GaAs), or indium phosphide (InP). In recent years, gallium nitride (GaN), aluminum nitride (AlN) and other wide-gap semiconductors have surfaced as new choices for both power electronics and visible-light generating optoelectronics. Gallium nitride material system gives rise to microwave transistors with high-electron mobility (necessary for high-speed operation), high breakdown voltage (necessary for high power), and thermal conductivity that is greater than GaAs, InP, or silicon, and thus suitable for use in high power applications. GaN is also used in manufacturing of blue and ultraviolet lasers and light-emitting diodes. In spite of the high-temperature performance (owing to its wide bandgap and high critical field), GaN electronic and optoelectronic devices are limited in performance due to relatively low thermal resistance of the substrates commonly used for growth of GaN. This deficiency is most pronounced in high-power microwave and millimeter-wave transistors and amplifiers where reduced cooling requirements and longer devices life, both benefiting from lower junction temperature, in critical demand. Similar need is exhibited in high power blue and ultraviolet lasers where several-micrometer-wide laser cavity stripe dissipates power into the chip though low thermal conductance materials.
The primary focus of this application is thermal management of high power microwave transistors, but the heat-flow management inventions introduced for microwave devices can be applied for heat management of semiconductor lasers, superluminescent diodes, and light-emitting diodes with departing from the spirit of the invention. The primary device on which the inventions will be described is the AlGaN/GaN high-electron-mobility transistor (HEMT).
The Structure of Conventional AlGaN/GaN HEMTs
Typical epilayer structure of AlGaN/GaN HEMT is shown in FIG. 10 (PRIOR ART). GaN is presently grown on several different substrates: sapphire, silicon, silicon carbide, aluminum nitride, single-crystal diamond, and GaN substrates. With the exception of GaN substrates, all other materials have lattice constants that differ from that of GaN and AlGaN. In order to epitaxially grow high-quality AlGaN alloys on top of substrates with lattice constant different from GaN or AlGaN alloy, it has been common practice in the industry to grow a layer or a combination of layers on top of the lattice-mismatched substrate in order to terminate the dislocations and produce a low-dislocation density epilayer on top of which growth of a high-quality active layers is possible. The active layers and resulting devices may be high-frequency transistors and/or optoelectronic devices such as laser diodes, light-emitting diodes, and super-luminescent diodes. The layers grown on top of the lattice-mismatched substrate are commonly referred to as nucleation layers or transition layers, and they can include any number of binary or ternary epitaxial layers followed by a suitably thick gallium-nitride layer added for achieving low dislocation density. Typical dislocation densities of GaN on silicon, silicon carbide, and sapphire epi-wafers for use in field-effect applications can be between 1E8 1/cm2 and 1E9 1/cm2. Defect density required for efficient operation of bipolar devices, such as, bipolar transistors and optoelectronic devices ranges from 1 E6 1/cm2 to 1 E8 1/cm2.
The epilayer structure of a typical AlGaN/GaN HEMT shown in FIG. 10 includes multiplicity of epilayers 9 disposed on top of a native substrate 1. The epilayers 9 can be divided into two functional parts: the transitions layers 8 and active layers 7. The transition layers 8 comprise of at least one layer, but typically a multiplicity of binary and ternary compound semiconductor epitaxial layers that are grown directly on top of the native substrate 1 and then followed by the buffer layer 3. The quality of the epilayers grown on the native substrate 1 improves past the layers 2 as the growth progresses and at some thickness indicated with dashed line 17, the crystal quality (defect density) of the buffer 3 becomes sufficient for high-crystal quality growth of the active layer 7. The active layers 7 comprise multiple epitaxial layers whose number, thickness, and material choices are designed and optimized to perform specific function of the electronic or optical device. For example, for an AlGaN/GaN HEMT, the active layers will typically comprise a barrier layer 6 on top of a layer structure 4 that may include a below-channel barrier to reduce drain-induced barrier lowering as is well known in the art. The barrier layer 6 may furthermore include a several nanometer thick layer of GaN and/or an AlN interlayer to improve the electron mobility in the two-dimensional electron gas 2DEG 5 as is also known in the art. The active layers 7 may comprise multiple layers of AlGaN or InGaN semiconductor alloys or GaN, AlN, InN or any other related material to realize the desired electrical performance of the HEMT. The boundary 17 between the active layers 7 and the transition layers 8 may not be a sharp line as indicated in FIG. 10, because the buffer layer 3 serves other purposes besides reducing the dislocation density. It is needed to electrically separate the transition layers from the electron gas 5 and its thickness may be increased to improve the device breakdown voltage. The exemplary HEMT shown in FIG. 1 will also feature contacts to the transistor denoted with 10 (source), 11 (gate), and 13 (drain). The source 10 and the drain 13 contacts will typically make ohmic contacts to the active layers 7, while the gate 11 will make a Schottky contact to the active layer 7. Additionally, individual HEMTs may be isolated from adjacent devices on the same wafer or chip using isolation trenches 12 or implantation (not shown) to form monolithically integrated circuits on the same chip. The operation of this transistor and device enhancements described above have been described in publicly available literature, such as, books by Rüdiger Quay titled “Gallium Nitride Electronics”, and Umesh K. Mishra and J. Singh titled “Semiconductor Device Physics and Design”, both books published by Springer in 2008.
GaN-based HEMTs are used for numerous high power applications owing to the high density of electrons in the 2DEG in GaN and the high-breakdown field which lead to high operating currents and voltages, higher than GaAs devices of similar geometry. The dominant heat generation in high-electron mobility transistors occurs in an area between the gate and the drain 15, close to the device surface. In this area, the energy of electrons accelerated with the high drain potential are first converted into optical phonons by electron-phonon scattering and then by phonon-phonon scattering into acoustic phonons which carry heat (heat conduction). Conventionally, the HEMT shown in FIG. 10 is mounted with the back of the substrate 1 down onto a heat sink: The back metallization 16 is attached to a heat sink (not shown in FIG. 1). The heat generated in the active layers of the transistor has to diffuse to the backside of the wafer and be carried away through the backside 16 by the heatsink and dissipated in the ambient. The temperature rise of the active layer relative to the ambient temperature for a given power dissipated by the device is referred to as the thermal resistance and is an essential design parameter for all electronic devices as it determines the device performance and its reliability. It is the objective high-power electronic and opto-electronic design is to minimize the thermal resistance of any device and thereby improve their performance over temperature and reliability.
Thermal resistance of commercial HEMTs with exemplary structure shown in FIG. 10 is dominated by the relatively low thermal conductivity of the layers in the immediate proximity of the active layer, namely, the thermal conductivity of the active layers 7 and the transition layers 8. More specifically, the nucleation layers 2 which are a part of transition layers 8 may comprise ternary compound semiconductor alloys which exhibit low thermal conductivity due to alloy scattering. Finally, some of the materials used commercially for the substrate 1 have low thermal resistance further contributing to the overall thermal resistance of the devices (eg. sapphire, silicon). The result of these materials and structure limitations is that conventional AlGaN/GaN field-effect transistors are limited thermally, but could be made better if its the thermal resistance could be somehow reduced.
There is a need in the industry to improve the thermal performance of AlGaN/GaN HEMTs and similar high-power electronic and optoelectronic devices. This need has spurred a number of investigations in integrating wide-bandgap device active materials with highly thermally conductive substrates by wafer bonding and/or direct growth of wide-gap materials